In conventional large-capacity inverters for driving industrial motors, thyristors that can easily provide a high breakdown voltage and allow flow of large current were used as switching devices. In middle- or small-capacity inverters, bipolar junction transistors were mainly used as switching devices. Later, IGBT (Insulated Gate Bipolar Transistor) has been used which exhibits both a high input impedance characteristic peculiar to MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and a low saturation-voltage characteristic peculiar to bipolar transistors. In recent years, the IGBT has been developed so as to provide a higher breakdown voltage and a larger current capacity, and are now employed in the field of thyristors. Because of a high current value to be handled by the IGBT, it is essential to protect the IGBT against overcurrent and overheat. Generally, drive circuits for driving these power devices are provided with overcurrent protection and overheat protection functions.
FIG. 5 is a circuit diagram showing a known drive circuit for an IGBT, which incorporates protection networks. In FIG. 5, an IGBT chip 100, a flywheel diode 200 and a drive circuit 300 are illustrated. The IGBT chip 100 principally consists of an IGBT 101, and a temperature detection diode 102 that is embedded in the chip and serves as a temperature sensor for detecting the junction temperature of the IGBT 101. The collector of the IGBT 101 is connected to the cathode of the flywheel diode 200, and the emitter is connected to the anode of the flywheel diode 200.
The drive circuit 300 includes a gate control unit 301 that is connected to the gate of the IGBT 101 and serves to control turn-on and turn-off of the IGBT 101, a comparator 302 for determining overcurrent or excess current of the IGBT 101, and a comparator 303 for determining excessively high temperature or overheat of the device. The comparator 302 has a non-inverting input terminal to which a junction between the sense emitter of the IGBT 101 and a resistor 304 is connected, and an inverting input terminal to which a reference voltage source 305 is connected. The comparator 303 has a non-inverting input terminal to which a reference voltage source 306 is connected, and an inverting input terminal to which a junction between a constant-current source 307 and the anode of the temperature detection diode 102 is connected.
The emitter of the IGBT 101, cathode of the temperature detection diode 102, negative terminals of the reference voltage sources 305, 306, and the resistor 304 are connected to the ground terminal (GND) of the drive circuit 300. An inductance L101 between the constant-current source 307 and the temperature detection diode 102, inductance L102 between the gate control unit 301 and the gate terminal of the IGBT 101, inductance L103 between the sense emitter of the IGBT 101 and the comparator 302, inductances L104, L105, L106, L107 on the ground GND represent inductances of internal wires.
In an overcurrent protection circuit of the IGBT 101, part of the emitter current of the IGBT 101 is taken out from the sense emitter, so that the sense emitter current flows through the resistor 304. The comparator 302 compares the terminal voltage that is produced across the resistor 304 due to the sense emitter current, with the voltage of the reference voltage source 305, and determines that overcurrent flows through the IGBT 101 when the terminal voltage of the resistor 304 due to the sense emitter current becomes higher than the voltage of the reference voltage source 305. In an overheat protection circuit of the IGBT 101, on the other hand, the comparator 303 compares the forward voltage of the temperature detection diode 102 through which a constant current flows from the constant-current source 307, with the voltage of the reference voltage source 306, and determines that the IGBT 101 is overheated when the forward voltage of the temperature detection diode 102 becomes lower than the voltage of the reference voltage source 306.
The IGBT 101 is turned on or off under control of the gate control unit 301 of the drive circuit 300. The portion of the IGBT 101 between the gate and the emitter behaves like a capacitor. Upon turn-on of the IGBT 101, therefore, the drive current charges the capacitor between the gate and the emitter, and flows from the gate to a negative terminal of a power supply (not shown), through the emitter and the ground GND. Upon turn-off, the charge stored between the gate and the emitter is discharged, and the discharge current flows from the gate to the emitter of the IGBT 101, through the gate control unit 301 and the ground GND. The drive current that flows upon turn-on and turn-off of the IGBT 101 is transient, and has a considerably large value on the order of ampere (A).
A plurality of sets or combinations each consisting of the drive circuit 300, IGBT 100 and the flywheel diode 200 as described above may be arranged in parallel with each other, along with a single direct-current power supply. In the case of a bridge circuit that use N-channel power devices to provide a polyphase inverter, for example, circuits for driving negative-side power devices may use a common direct-current power supply. In the case of a bridge circuit that use P-channel power devices to provide a polyphase inverter, circuits for driving positive-side power devices may use a common direct-current power supply. The following example illustrates two drive circuits corresponding to two phases on the negative side of a bridge circuit that uses N-channel power devices to provide a three-phase inverter.
FIG. 6 is a view showing an example of connection of two drive circuits that share a single power supply. In FIG. 6, the same reference numerals as used in FIG. 5 are used for identifying the corresponding constituent elements, of which no detailed description will be provided. For the sake of brevity, the circuits for overcurrent protection and overheat protection are not illustrated in FIG. 6 nor explained in the following description.
The gate of the IGBT 101 is connected to the drive circuit 300, and the collector is connected to a load terminal V, while the emitter is connected to a load terminal N. In internal wires through which the main current of the IGBT 101 flows between the load terminal V and the load terminal N, an inductance 108 exists on the side of the collector of the IGBT 101, and an interphase inductance 109 exists between the emitter of the IGBT 101 and the emitter of the adjacent IGBT 101a, while an inductance L110 exists between the emitter of the IGBT 101a and the load terminal N. The drive circuit 300 is connected to the positive and negative terminals of a dc power supply 400, and inductances L104, L105, L106, L107 exist in an internal wire that extends from the ground GND to the negative terminal of the dc power supply 400. Similarly, IGBT 101a and drive circuit 300a are provided in the circuit for another phase as shown in the lower part of FIG. 6, and the same dc power supply 400 as used for the drive circuit 300 is connected to the drive circuit 300a. Also, inductances L104a, L105a, L106a, and L107a exist in an internal wire on the ground GND of the drive circuit 300a.
When the upper drive circuit 300 supplies drive current to the IGBT 101, the IGBT 101 is turned on, and load current I.sub.ON flows from the load terminal V to the load terminal N, through the inductances L108, IGBT 101, and the inductances L109, L110. Similarly, when the lower drive circuit 300a supplies drive current to the IGBT 101a, the IGBT 101a is turned on, and load current flows from the load terminal U to the load terminal N, through the IGBT 101a. In this manner, two IGBT switching circuits that use the common dc power supply 400 operate independently of each other.
In the circuit as shown in FIG. 5, however, transient drive current flows upon turn-on and turn-off of the IGBT 101, and transient voltages are produced across the inductances L102, L104, L105, L106, L107 of the internal wires on the current loop. The transient voltages cause variations in the operating points of the protection circuits, which may result in malfunction of the protection circuits. The mechanism that causes the malfunction will be described below in detail.
FIG. 7 is a view useful in explaining flow of drive current upon turn-on of the IGBT 101, and FIG. 8 is a view useful in explaining flow of drive current upon turn-off. When the IGBT 101 is turned on upon application of a voltage to between the gate and the emitter thereof, charging current that provides drive current I.sub.DON transiently flows into a capacitor that is assumed to be present between the gate and the emitter, as shown in FIG. 7. At this time, the drive current I.sub.DON causes a transient voltage to be produced across each of the inductances L104, L105, L106, L107 on the path through which the drive current I.sub.DON flows, in particular, those inductances that exist on the ground GND that provides a reference potential for the overcurrent protection circuit and overheat protection circuit. With respect to the overcurrent protection circuit, for example, if a transient voltage is produced across the inductance L106 on the ground GND due to flow of the drive current I.sub.DON, the potential of the negative terminal of the reference voltage source 305 becomes lower than the potential on the ground side of the resistor 304, by an amount corresponding to the transient voltage, and the voltage of the reference voltage source 305 is substantially reduced by the amount corresponding to the transient voltage. As a result, the operating point of the comparator 302 is changed, and the overcurrent protection circuit may fail to perform appropriate protecting operations, or may malfunction during normal operations. In the overheat protection circuit, too, if the drive current I.sub.DON causes a transient voltage to be produced across the inductance L104 on the ground GND, the transient voltage substantially reduces the voltage of the reference voltage source 306 in a similar manner, resulting in a change in the operating point of the comparator 303. Consequently, the overheat protection circuit may fail to perform proper protecting operations. Upon turn-off of the IGBT 101, on the other hand, charge stored between the gate and the emitter is discharged, and discharge current provides drive current I.sub.DOFF, which transiently flows through the inductances L102, L107, L106, L105, L104, as shown in FIG. 8. In this case, too, a transient voltage appears across the inductance L106 on the ground GND, and the potential of the negative terminal of the reference voltage source 305 becomes higher than the potential of the ground side of the resistor 304 by an amount corresponding to the transient voltage. As a result, the voltage of the reference voltage source 305 is substantially increased by the amount corresponding to the transient voltage, and the operating point of the comparator 302 is changed. At the same time, a transient voltage is produced across the inductance L104, which results in a substantial increase in the voltage of the reference voltage source 306, and a change in the operating point of the comparator 303. Thus, upon either turn-on or turn-off of the IGBT 101, the protection circuits cannot perform normal protecting operations.
In the circuit arrangement as shown in FIG. 6, when load current I.sub.ON flows from the load terminal V to the load terminal N upon turn-on of the IGBT 101, for example, a transient voltage is produced across each of the inductances L108, L109, L110. In particular, the transient voltage produced across the inductance L109 causes transient circulation current I.sub.LOOP to flow from the inductance L109 through a loop circuit that includes the inductances L104, L105, L106, L107 on the ground GND of the drive circuit 300 and the inductances L107a, L106a, L105a, L104a on the ground GND of the drive circuit 300a, and then return to the inductance L109. This transient circulation current I.sub.LOOP causes a transient voltage to be produced across each of the inductances L104, L105, L106, L107 and inductances L104a, L105a, L106a, L107a, and the transient voltages thus produced may cause changes in the reference voltage and detection voltage of the overcurrent protection circuit, or the reference voltage of the overheat protection circuit. Thus, the protection circuits may fail to perform normal protecting operations, or may malfunction during normal operations. Upon turn-off of the IGBT 101, too, a negative transient voltage is produced across the inductance L109 due to a reduction in the load current I.sub.ON, and reverse circulation current--I.sub.LOOP flows through the loop circuit as indicated above, whereby different potentials appear at different locations on the common ground GND in the drive circuits, thus causing malfunction of the protection circuits.